Prolactin (PRL) stimulates the growth and function of target tissues such as the mammary gland (MG) and the prostate and may contribute to the development of breast and prostate cancer. However, no clear correlation has been found between circulating PRL levels (measured by radioimmunoassay; RIA) and the incidence and risk of mammary tumors in women. Such lack of correlation could be explained by the existence of bioactive forms of PRL with low immunoactivity. We have isolated a 16K fragment of PRL, generated in vitro by the MG, with a high bioassay: RIA ratio. Using a specific RIA for 16K PRL that we have developed, we have found that significant amounts of 16K PRL are present in the circulation of rats in different physiological states. Knowledge about the functional relevance of the proteolysis of PRL into the 16K fragment by the MG may add to the understanding of the role of PRL in the regulation of normal and cancerous mammary growth. To analyze whether 16K plays a role in MG function, we will: A. Test if for lactogenic activity by measuring casein synthesis by rat mammary cells. B. Determine which tissues can produce cleaved and 16K PRL in vitro and whether the rate of production varies with the physiological state of the animal. C. Survey various tissues for the presence of specific receptors for the 16K PRL to obtain information on potential target organs of the fragment. D. Determine whether 16K in serum is monomeric, aggregated or carrier bound. E. Measure by RIA the 16K levels in tissues (pituitary, liver and MG) and serum of rats whose MG are in different physiological states of growth and function, and in rats bearing chemically- induced (DMBA) mammary tumors. 16K PRL may also be produced inside the stomach of rat pups from the PRL in milk. We found that enzymes required for the proteolysis of PRL into the 16K fragment are present in milk, and dependent on an acid pH. To study whether 16K PRL is present and exerts some effect in the nursing young, we will: A. Determine the 16K levels in stomach milk and in the circulation of suckling rats. B. Analyze the effect of passive immunization against 16K PRL on the rate of growth of infant rats. C. Determine whether specific 16K receptors are present in various tissues of rat pups.